I was able to connect my linux box (intel HD3000/sandy bridge) to my TVS in 1080i50 mode.the only thing I have is that the ATEM sends tons of modes in the EDID when it should really send only one (the currently running mode).

On certain Windows models, you might have to choose 25hz instead of 50hz for 50i. We have an old Windows laptop here which we can hook it up to the ATEM. However, it only can run as 1080 @ 25hz and somehow it works well. I'm not too sure if this will work for you.

For Mac, it's more straightforward as all you need to choose is 1080 and 50hz.